error while creating a thread with a class function

Question

In below code, at thread t(&Fred::hello) i am getting an error that the term does not evaluate to a function taking 0 arguments. What is the issue?

#include <iostream>
#include <thread>

using namespace std;

class Fred
{
public:

virtual void hello();

};

void Fred::hello()
{
cout << "hello" << endl;
}

int main()
{
thread t (&Fred::hello);
t.join();

return 0;
}

juanchopanza · Accepted Answer · 2013-05-10 15:37:47Z

4

A non-static member function of class T needs to be called on an instance of T, and takes an implicit first parameter of type T* (or const, and/or volatile T*).

So

Fred f;
f.hello()

is equivalent to

Fred f;
Fred::hello(&f);

So when you pass a non-static member function to a thread constructor, you must pass the implicit first argument too:

Fred f;
std::thread t(&Fred::hello, &f);

answered May 10, 2013 at 15:37

juanchopanza

228k35 gold badges421 silver badges493 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Jimm Over a year ago

That is great explaination. But is there a version of thread constructor which can use emplace semantics? It seems a waste that i have to do pass &f, only for the thread constructor to copy it again?

juanchopanza Over a year ago

@Jimm the thread constructor will only copy a pointer, not a Fred object.

Jimm Over a year ago

I just tried passing f and not &f, such as std::thread t(&Fred:hello, f) and it seems to be working. Plus constructor is called only once. In this case, thread constructor seems to be avoiding copying of f.

Jimm Over a year ago

hmm never mind, i was wrong. When i pass just f, the copy constructor is being called 3 times. Any idea why 3 times?

Collectives™ on Stack Overflow

error while creating a thread with a class function

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related